Re: Error from the foreign RDBMS on a foreign table I have no privilege on

On Wed, 2022-06-08 at 19:06 +0900, Etsuro Fujita wrote:
> On Wed, Jun 8, 2022 at 2:51 PM Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> wrote:
> > At Wed, 08 Jun 2022 07:05:09 +0200, Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> wrote in
> > > diff --git a/doc/src/sgml/postgres-fdw.sgml b/doc/src/sgml/postgres-fdw.sgml
> > > index b43d0aecba..b4b7e36d28 100644
> > > --- a/doc/src/sgml/postgres-fdw.sgml
> > > +++ b/doc/src/sgml/postgres-fdw.sgml
> > > @@ -274,6 +274,14 @@ OPTIONS (ADD password_required 'false');
> > >         but only for that table.
> > >         The default is <literal>false</literal>.
> > >        </para>
> > > +
> > > +      <para>
> > > +       Note that <command>EXPLAIN</command> will be run on the remote server
> > > +       at query planning time, <emphasis>before</emphasis> permissions on the
> > > +       foreign table are checked.  This is not a security problem, since the
> > > +       subsequent error from the permission check will prevent the user from
> > > +       seeing any of the resulting data.
> > > +      </para>
> > >       </listitem>
> > >      </varlistentry>
> >
> > Looks fine.  I'd like to add something like "If needed, depriving
> > unprivileged users of relevant user mappings will prevent such remote
> > executions that happen at planning-time."
>
> I agree on that point; if the EXPLAIN done on the remote side is
> really a problem, I think the user should revoke privileges from the
> remote user specified in the user mapping, to prevent it.  I’d rather
> recommend granting to the remote user privileges consistent with those
> granted to the local user.

I don't think that is better. Even if the local and remote privileges are
consistent, you will get an error from the *remote* table access when trying
to use a foreign table on which you don't have permissions.
The above paragraph describes why.
Note that the original complaint against oracle_fdw that led to this thread
was just such a case.

Yours,
Laurenz Albe